Providing a way to make a custom element matchable to the following pseudo classes would be helpful.

:checked, :indeterminate, :in-range, :out-of-range, :required, :optional, :read-only, :read-write

Usecase:
A web author makes a checkbox-like custom element, and want to support :checked pseudo class like the built-in <input type=checkbox>.

We discuss a similar feature at #738. Its selector syntax is something like :state(checked), and it's not compatible with built-in elements.

Proposal and feedbacks:
The initial idea was #738 (comment) .
There is a concern about difficulty of correct usage; #738 (comment)